The present invention relates to a fabrication method of low-loss plastic optical fibers consisting of a core of a polymer which is prepared from methyl methacrylate as the principal component and a cladding of a synthetic macromolecular compound having a lower refractive index than that of the core.
Heretofore, it has been well known to fabricate a plastic optical fiber having a concentric core-cladding structure consisting of a core component made of a synthetic macromolecular compound with excellent transparency represented by polystyrene or polymethyl methacrylate and a cladding component made of another synthetic macromolecular compound having a lower refractive index than that of the core component. And it has also been well known that the incident light introduced from one end of such optical fiber as set forth above is totally reflected inside the fiber along the longitudinal direction thereof to effect the transmission of the light. The matter to be taken into consideration in view of fabricating such kind of plastic optical fiber is that such a factor for increasing attenuation of light due to the absorption or scattering of the light in case of transmitting such light through the interior of the fiber should be minimized. A so-called plastic optical fiber made of synthetic macromolecular compounds has such advantages as a lighter weight and superior flexibility, besides being easy to increase the numerical aperture than those of an optical fiber fabricated from an inorganic glass which has heretofore been known. On the contrary, such plastic optical fibers have a disadvantage that the degree of attenuation of light transmitting through the interior of the fiber is remarkable than that of a conventional inorganic glass optical fiber. In this respect, the present invention directs to reduction of the degree of attenuation of light in a plastic optical fiber produced from macromolecular compounds.
Such plastic optical fiber consists of a polymer for the core part and another polymer, having a lower refractive index than that of the former polymer, for the cladding part. In a conventional fabrication of plastic optical fibers, a clear polymer such as polystyrene, polymethyl methacrylate or the like has been employed as the core material, whilst another polymer having a lower refractive index than that of the clear polymer, more specifically, polymethyl methacrylate or the like in case where polystyrene is adopted as the core material, and fluorine polymer in case where polymethyl methacrylate is adopted as the core material has been utilized. The fabrication of a plastic optical fiber is carried out by double extrusion molding in which a cladding is applied around the outer periphery of a core fiber so as to form a covering layer for the core fiber simultaneously with the formation of the core fiber from the core material (such process being hereinafter referred to as "double spinning process"), or in accordance with a coating process in which a core fiber which has previously been formed is coated with a cladding to cover the outer periphery of the core fiber.
A plastic optical fiber has a larger diameter, large numerical aperture, and very superior flexibility than those of such optical fiber in which inorganic glass is utilized as the core material, and accordingly, the plastic optical fiber has such advantage that coupling efficiency with respect to the light source is significantly elevated, so that joining between fibers becomes very easy, but such a disadvantage that there is a larger transmission loss, on the contrary.
The transmission loss of a plastic optical fiber being now available on the market is about 300 dB/km, even if the most favorable wavelength band is selected (at wavelength of 570 nm or 650 nm), thus it is desired to lower transmission loss of such plastic optical fiber. Under these circumstances, the present inventors have variously studied upon a series of steps from the preparation of a core and clad materials to the formation of the core fiber in a conventional fabrication method in order to fix a cause for accompanying such a high transmission loss in a conventional plastic optical fiber, and as a result the following knowledge has been obtained.
Namely, in a conventional fabrication method of a plastic optical fiber, the core and cladding materials are generally synthesized in accordance with suspension polymerization method, and the so synthesized polymers are supplied to a device for fabricating such plastic optical fiber. It is generally recognized that suspension polymerization method is a method by which a polymer with a high purity can be obtained as an industrial method for synthesizing a macromolecular compound, but there is such a disadvantage in that since suspension polymerization method requires a large amount of water, the resulting polymer is easily contaminated with optical foreign materials contained in such water. Furthermore, there is also such disadvantage that a possibility of the contamination of the polymer by optical foreign materials is very remarkable in the course of the dehydration step thereof.
In addition, a pelletizing or preforming step for the resulting polymer is required for forming or melt-spinning the polymer. Besides there is also such a fear that the polymer is contaminated by optical foreign materials in the course of the pelletizing step for the polymer or a feeding step of such pelletized polymer to a fiber fabrication apparatus, or that the polymer is oxidized by air, because a polymer preparation apparatus is separately located from the fiber fabrication apparatus in most cases. Therefore, it is considered that the disadvantage of a high transmission loss in a conventional plastic optical fiber as mentioned above can be dissolved, if such obstacles are removed.
As a method for improving optical transmission characteristics of plastic optical fibers, a fabrication method in which the biacetyl content in methyl methacrylate is reduced, transition metallic ions are decreased, and further other particulate matters are removed by the filtration thereof, whereby an improved plastic optical fiber is obtained (see U.S. Pat. No. 4,161,500 and Japanese Laid-open Patent Application No. 65,555/1979 corresponding thereto), or fabrication method in which a core component is subjected to bulk polymerization, and then the separation of volatile materials containing a residual unreacted monomer as the principal constituent in the polymer is successively carried out, thereby to prepare an improved plastic optical fiber (see Japanese Laid-open Patent Application Nos. 83,046/1975 and 83,047/1975 as well as U.S. Pat. No. 3,993,834 corresponding thereto) has been proposed.
In connection with these methods, it is disclosed in U.S. Pat. No. 4,161,500 that attenuation of light accompanied with the existence of particulate materials can be improved on the basis of such fact that vinyl monomer contains substantially no particulate materials, i.e., preferably such vinyl monomer merely contains 100/mm.sup.3 or less particulate materials. According to the present inventors' knowledge, however, even if around 10/mm.sup.3 of particulate materials exist in such monomer, attenuation of light is significant, so that even the existence of 2/mm.sup.3 of particulate materials in the monomer is not sufficient in order to obtain a low-loss plastic optical fiber. Namely, the existence of 100/mm.sup.3 of particulate materials means the existence of around 20,000 particulate materials per 1 m fiber length in case, for example, of a plastic optical fiber having 0.5 mm diameter. As a consequence, if 1 particulate materials brings about loss of around 1/1,000 dB/m, it becomes loss of 20 dB/m in 1 m length fiber. As described above, it was confirmed that the existence of around 10/mm.sup.3 particulate materials caused a significant optical transmission loss, so that even the existence of 2/mm.sup.3 particulate materials in a monomer was not sufficient in order to fabricate a low-loss plastic optical fiber.
In the method according to U.S. Pat. No. 4,161,500 in which monomers are polymerized in a sealed system, the removal of dust or impurities etc. incorporated at the time of addition of a polymerization initiator and chain transfer agent to the monomers is attempted by the use of a filter having around 0.2-1 .mu.m opening diameter in the following step, but such particulate materials still considerably remain in the monomers, and around 2/mm.sup.3 particulate materials clearly exist therein in spite of such expression that substantially no particulate materials contains. Furthermore, contamination of a granular article by dust or the like cannot be avoided in also the case where a ramextrusion material is polymerized in a sealed system, and then the resulting granular article is taken out and transferred to a spinning device. For this reason, attenuation amount of the plastic optical fiber fabricated in accordance with this method is merely a value of around 300 dB/km (at wavelength of 656 nm).
Moreover, in U.S. Pat. No. 4,161,500 which states that impurities, particularly, impurity ions such as transition metallic ions or the like in the polymer should be 500 ppb or less, preferably 100 ppb or less, if there exists 10 ppb of cobalt ion, a considerable increase of loss such as 50 dB/km causes at wavelength of 630 nm, or if 100 ppb of nickel ion exists, there causes more significant increase of loss such as 33 dB/km at wavelength of 850 nm.
Further, in also a continuous bulk polymerization process (see Japanese Laid-open Patent Application Nos. 83,046/1975 and 83,047/1975, or U.S. Pat. No. 3,993,834), the methods for the purification of a monomer as well as the addition of a polymerization initiator and chain transfer agent (molecular weight modifier) into the monomer are not suitable, so that contamination of the monomer by dust or impurities cannot be avoided. Although such process can decrease scattering loss due to particulate matters or absorption loss due to impurities up to a certain extent, the minimum value of attenuation amount is only a value of around 300 dB/km (at wavelength of 656 nm).
On one hand, as to a cladding component polymer, since the light transmitting through a core fiber is transmitted while being totally reflected on the core-cladding boundary surface, if the light is absorbed or scattered by the cladding component in the aforesaid boundary surface, permeability of the optical fiber is remarkably reduced. Especially, if the cladding component has crystallizability and opacity, scattering of the light is remarkable and in addition, even in case of such cladding component in which micro voids generates in the boundary surface, scattering of the light is outstanding. As clad component polymers, a copolymer of vinylidene fluoride and tetrafluoroethylene (see U.S. Pat. No. 3,930,103 or Japanese Patent Application Publication No. 21,660/1978 corresponding thereto) or fluoroalkyl methacrylate polymer (see U.K. Patent No. 1,037,498 or Japanese Patent Application Publication No. 8,978/1968 corresponding thereto) etc. has conventionally been known. In this case, since some crystallizability remains in the copolymer of vinylidene fluoride and tetrafluoroethylene, it results in lowering of optical permeability due to scattering of light in the core-cladding boundary surface. On the other hand, though fluoroalkyl methacrylate polymer is non-crystalline, the polymer has such disadvantages in that the fluoroalkyl methacrylate polymer containing fluoroalkyl groups having a sufficient adhesion to a core fiber has a low softening point, and that the fluoroalkyl methacrylate polymer containing fluoroalkyl groups having a relatively high softening point exhibits not necessarily excellent adherence to the core fiber. Besides, there is such a problem in that voids remain in the core-cladding boundary surface, because the polymerization condition for such fluoroalkyl methacrylate is not suitable. In case where there is an inferior adhesion of a cladding polymer to the core fiber, or remaining of voids in the core-cladding boundary surface, scattering of light in the boundary surface increases, whilst the optical permeability remarkably decreases.
In order to improve these disadvantages, such a method in which an unsaturated polymerizable compound is copolymerized as the third component with the copolymer of vinylidene fluoride and tetrafluoroethylene within a certain range, whereby the low crystallizability, transparency, adhesion to the core component and the like of the vinylidene fluoride-tetrafluoroethylene copolymer are improved has been proposed (Japanese Laid-open Patent Application No. 80,758/1979). Further, a method in which the structure of the fluoroalkyl group in fluoroalkyl methacrylate polymer is modified to improve the softening point of the polymer has been proposed (see Japanese Patent Application Publication Nos. 8,321/1981, 8,332/1981 and 8,323/1981). Among these methods, a method in which the ternary copolymer is employed (Japanese Laid-open Patent Application No. 80,758/1979) has such a problem in that crystallizability still remains, so that a clear polymer cannot be obtained, and a method in which the structure of fluoroalkyl group is modified (see Japanese Patent Application Publication Nos. 8,321/1981, 8,322/1981 and 8,323/1981) also accompanies such a problem in that such fluoroalkyl methacrylate polymer having a high softening point together with an excellent adhesion cannot be yet obtained. For these reasons as described above, plastic optical fibers fabricated in accordance with the aforesaid methods merely exhibit a value of around 78% white transmittance per 50 cm of an optical fiber in respect of optical transmission characteristics.